(1) Field of the Invention
The present invention generally relates to an evaporated fuel purge control apparatus, and more particularly to an evaporated fuel purge control apparatus for an internal combustion engine in which the flow of evaporated fuel being fed from a canister into an intake passage of the engine is controlled using a plurality of purge control valves arranged in parallel in a passage between the canister and the intake passage.
(2) Description of the Related Art
In an internal combustion engine, an evaporated fuel purge control system is provided. In this evaporated fuel purge control system, evaporated fuel from a fuel tank is stored in a canister, and the stored fuel is fed from the canister to an intake passage of the engine through a purge control valve arranged in a passage between the canister and the intake passage. The flow of evaporated fuel from the canister to the intake passage is controlled by the purge control valve under prescribed operating conditions of the engine.
Certain types of the evaporated fuel purge control systems have been proposed in which two purge control valves are arranged in parallel in a passage between a canister and an intake passage. For example, Japanese Laid-Open Patent Publication No. 62-233466 discloses an evaporated fuel purge system of this type. In the system of this publication, a master control valve and a slave control valve are arranged in parallel in order to supply a suitable amount of evaporated fuel to the intake passage at a suitable timing. Also, there has been proposed an evaporated fuel purge control apparatus in which a duty cycle control valve is provided in order to accurately control the flow of the evaporated fuel into the intake passage.
However, if an evaporated fuel purge control apparatus including a plurality of purge control valves arranged in parallel in a passage between the canister and the intake passage is provided in an internal combustion engine, the engine will be subjected to a pulsating flow of evaporated fuel being fed into the intake passage because the valves are opened and closed at duty cycles and phases which are equal to each other. Also, it is likely that the evaporated fuel is supplied to only a specific cylinder of the engine, and the turbulence of the air fuel ratio will occur in the engine.
In addition, Japanese Laid-Open Patent Publication No. 61-105601 discloses a pulse width modulation (PWM) fluid flow controlling method. In this method, the switching operations of two solenoid valves arranged in parallel are controlled using PWM signals having the duty cycle equal to each other and having the same phase so as to drive an actuator by the flow of operating fluid.
However, in the conventional method disclosed in the above mentioned publication, there is a problem in that the number of switching operations of the two control valves is relatively large. Therefore, an evaporated fuel purge control apparatus with a plurality of purge control valves to which the above method is applied will be noisy and less durable.